The objectives of this investigation are: (1) To further explore the relationship between adrenergic nervous activity and ventricular rhythm disturbances induced by digitalis. (2) To determine the relative contribution of neural and myogenic factors to the genesis of digitalis-induced arrhythmia. (3) To determine the relative importance of drug action on adrenergic nerves and cardiac muscle in preventing digitalis-induced arrhythmias. Several different lines of investigation will be employed. The effect of adrenergic nervous activity, drugs and total cardiac denervation on the temporal dispersion in recovery of excitability will be investigated. If nonuniformity of excitability increases after arrhythmogenic influences, then antiarrhythmic procedures may result from the action of drugs to restore more uniform activity in the nerves and/or muscle. Arrhythmias will be produced by several digitalis materials since there appears to be differences in the action of glycosides to produce rhythm disturbances. These differences may reside in different capacities to affect neural and muscle structures and to release catecholamines from the adrenal medulla. The studies will involve preparations such as the newborn rabbit in which susceptibility to arrhythmia at various stages of adrenergic neuronal development can be explored. The capacity of an agent to directly depress the myocardium will be determined by recording its effect on transmembrane action potential and ionic fluxes. The capacity of digitalis materials to release amines form the adrenal medulla will be characterized.